After joining NASA, Slayton was selected to pilot the second U.S. manned orbital spaceflight, but was grounded in 1962 by atrial fibrillation, an irregular heart rhythm. He then served as NASA's Director of Flight Crew Operations, making him responsible for crew assignments at NASA from November 1963 until March 1972. At that time he was granted medical clearance to fly, and was assigned as the docking module pilot of the 1975 Apollo–Soyuz Test Project, at age 51 becoming the oldest person to fly in space at the time. This record was surpassed in 1983 by 54-year-old William E. Thornton, 59-year-old (and fellow ASTP crew member) Vance Brand in 1990, 61-year-old Story Musgrave in 1996, and in 1998 by Slayton's fellow Project Mercury astronaut John Glenn, who at the age of 77 flew on Space Shuttle mission STS-95.

Slayton died at the age of 69 on June 13, 1993, from a malignant brain tumor.

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Slayton was born on March 1, 1924, on a farm near Sparta, Wisconsin, to parents Charles Sherman Slayton (1887–1972) and Victoria Adelia Slayton (née Larson; 1895–1970).[2] He was of English and Norwegian descent. In 1929, a childhood farm equipment accident left him with a severed left ring finger. He attended elementary school in Leon, Wisconsin, and graduated from Sparta High School in 1942.

In 1959, Slayton was one of 110 military test pilots selected by their commanding officers as candidates for the newly formed National Aeronautics and Space Administration's Project Mercury, the first U.S. manned space flight program. Following a series of physical and psychological tests, NASA selected Slayton to be one of the original group of seven Mercury astronauts.[2]

During the summer of 1961, Slayton was diagnosed with an erratic heart rate (idiopathicatrial fibrillation).[6] Despite his best efforts to overcome this condition, including exercise and lifestyle changes, none of it had any effect.

Slayton was scheduled to fly in May 1962 on the program's second orbital flight (Mercury-Atlas 7), which he would have named Delta 7 (after the fourth letter in the Greek alphabet since this was the fourth manned Mercury flight, and "7" designating the seven astronauts), but because of uncertainty over his heart condition, the flight was given to Scott Carpenter instead and Slayton was grounded from flights completely the following September.[3] He ended up as the only one of the Mercury Seven to not fly a Mercury mission.

When NASA grounded Slayton, the Air Force followed suit. From September 1, 1962 until November 1963, he obtained the unofficial title of "chief astronaut" when he was assigned as Coordinator of Astronaut Activities, a position later re-designated as Chief of the Astronaut Office. Despite forfeiting a pension that he would have earned following twenty combined years of active duty, Air Guard and Reserve service in 1964, Slayton resigned his Air Force commission in 1963 and continued to work for NASA in a civilian executive capacity, first as Assistant Director of Flight Crew Operations until 1966, and then as Director of Flight Crew Operations.[8] As Director, he oversaw the activities of the astronaut office (managed by Chief of the Astronaut Office Alan Shepard, also grounded due to Ménière's disease), the aircraft operations office, the flight crew integration division, the crew training and simulation division, and the crew procedures division. He had the decisive role in choosing the crews for the Gemini and Apollo programs, including the decision of who would be the first person on the Moon.

While grounded, Slayton took several measures to attempt to be restored to flight status, including a daily exercise program, quitting cigarette smoking and coffee, and drastically reducing consumption of alcoholic beverages. He also took massive doses of vitamins, and for a time took daily doses of quinidine, a crystalline alkaloid.

In July 1970, the fibrillation ceased, and a comprehensive review of his medical status by NASA's director of life sciences and the Federal Aviation Agency resulted in the full restoration of his flight status in March 1972. Slayton celebrated with an hour of aerobatic maneuvers in a NASA T-38 jet trainer.[2]

After he was restored to flight status, Slayton was selected in February 1973 as docking module pilot for the Apollo–Soyuz Test Project, a docking between an American Apollo spacecraft and a Soviet Soyuz spacecraft. The American crew immediately began an intensive two-year training program, which included learning the Russian language and making frequent trips to the USSR, where astronauts trained for weeks at Star City, the cosmonaut training center near Moscow. Slayton resigned as Director of Flight Crew Operations in February 1974.[2]

On July 17, 1975, the two craft joined up in orbit, and astronauts Slayton, Thomas P. Stafford and Vance D. Brand conducted crew transfers with cosmonauts Alexey Leonov and Valeri Kubasov. At the end of the flight, an erroneous switch setting led to noxious nitrogen tetroxide fumes from the command module's RCS thrusters being sucked into the cabin during landing, and the crew was hospitalized as a precaution in Honolulu, Hawaii, for two weeks. During hospitalization, a lesion was discovered on Slayton's lung and removed. It was determined to be benign, but he would have almost certainly been grounded from ASTP if this had been discovered before the flight.

Following the ALT program, Slayton served as Manager for Orbital Flight Test, directing orbital flight mission preparations and conducting mission operations. He was responsible for OFT operations scheduling, mission configuration control, preflight stack configuration control, as well as conducting planning reviews, mission readiness reviews, and postflight mission evaluations. He was also responsible for the Shuttle Carrier Aircraft program.[2]

Although Slayton expressed his hope of flying on a Shuttle mission, new NASA management did not favor him, making it clear that they considered him part of the past and that they planned to recruit a new young group of astronauts for the Shuttle era.

Slayton retired from NASA in 1980, but retained unofficial ties as a consultant until 1982. With 23 years of service, he set a record still unbroken as of 2017 for the longest-serving astronaut in history, despite only flying in one mission and being grounded from flights for years. After retirement, he served as president of Space Services Inc., a Houston-based company earlier founded to develop rockets for small commercial payloads. He served as mission director for a rocket called the Conestoga, which was successfully launched on September 9, 1982, and was the world's first privately funded rocket to reach space.[9] Slayton also became interested in aviation racing. In addition to serving as a consultant to some aerospace corporations, he was President of International Formula One Pylon Air Racing and Director of Columbia Astronautics. He also served on the Department of Transportation's Commercial Space Advisory Committee.[2]

Slayton penned an autobiography with space historian Michael Cassutt entitled Deke!: U.S. Manned Space from Mercury to the Shuttle.[10] As well as Slayton's own astronaut experiences, the book describes how he made crew choice selections, including choosing the first person to walk on the Moon. Numerous astronauts have noted that only when reading this book did they learn why they had been selected for certain flights decades earlier.

Slayton's name also appears with three other co-authors, including fellow astronaut Alan Shepard, on the book Moon Shot: The Inside Story of America's Race to the Moon, published in 1994.[11] The book was also made into a documentary film of the same name. Slayton died before either Moon Shot project was finished or released, and the book did not receive any input from him. However, the film was narrated from Slayton's point of view (voiced by Barry Corbin) and includes a brief tribute to him at the end.

Slayton was a close friend of fellow astronaut Gus Grissom.[12] He married Marjorie "Marge" Lunney (1921–1989) in May 1955, and they had one son, Kent Sherman, born April 8, 1957.[13]:345 They eventually divorced, and Slayton later married Bobbie Belle Jones (1945–2010) in 1983. They remained married until his death.[13]:350 His hobbies were hunting, fishing, shooting, and airplane racing.[14]

Shortly after he moved to League City, Texas, in 1992, Slayton was diagnosed with a malignant brain tumor. He died from the illness, at the age of 69, on June 13, 1993.[15] He was cremated and his ashes scattered over his family farm in Wisconsin.

The Texas Oncology-Deke Slayton Cancer Center (located on Medical Center Blvd. in Webster, Texas) was named in his honor in 2000.[21]

The main stretch of road in League City, Texas, FM 518, was renamed Deke Slayton Highway.[22]

The Deke Slayton Memorial Space & Bicycle Museum in Sparta, Wisconsin, was named in his honor.[23] The Slayton biographical exhibit includes his Mercury space suit, his Ambassador of Exploration Award, which showcases a lunar sample, and more. In nearby La Crosse, Wisconsin, an annual summer aircraft air show, the Deke Slayton Airfest, has been held in his honor, featuring modern and vintage military and civilian aircraft, along with NASA speakers.